Streptococcal pathogens continue to evade concerted efforts to decipher clear-cut virulence mechanisms, although numerous genes have been implicated in pathogenesis. A single species can infect a diversity of tissues, suggesting the expression of specific virulence factors based on the local tissue environment or stage of infection. Our long-range goal is to identify the interactions that occur between the host and pathogen that lead to activation of virulence mechanisms and contribute to specific streptococcal disease states. The objective of this application is to characterize specific virulence mechanisms utilized within various tissues in vivo by employing a unique animal model, the zebrafish (Danio rerio). We will accomplish this by studying infection by two streptococcal species that represent two forms of streptococcal disease: a natural pathogen of both fish and humans, Streptococcus iniae and a human-specific pathogen, Streptococcus pyogenes. While S. iniae primarily causes systemic disease in the zebrafish following intra-muscular injection, S. pyogenes causes a locally spreading necrotic disease confined to the muscle. By studying pathogens that are virulent for both fish and humans and that mediate disease states in the zebrafish that are identical to those found in human streptococcal infections, we will be able to identify common virulence strategies shared by a number of Gram positive pathogens. The central hypothesis is that streptococcal pathogens respond to their host by initiating specific virulence mechanisms based on the local tissue environment or host-specific factor expressed within that tissue. We propose to: (1) identify and characterize bacterial proteins that interact with the host in vivo to cause specific disease states; (2) characterize the role in pathogenesis of proteins previously implicated in virulence; and (3) identify the factors required for initial colonization by streptococcal pathogens.